U.S. Pat. No. 3,762,514 to Freitag (issued October, 1973) discloses an adjustable-length support column designed for a vehicle seat to reduce resiliency under overload condicitons, such as in the event of a vehicle collision. The column includes a piston for attachment to a movable part and a floating piston mounted in separate parts of a cavity divided by a fixed wall. The wall has a normally-open throttle valve designed to close when the hydraulic pressure in the first piston cavity part suddenly increases, thereby substantially reducing column resiliency under collision conditions. The floating piston rests on an pneumatic cushion and the first piston includes a longitudinal through bore which respectively provide resiliency and shock-absorbing effects.
However, the Freitag unit may not be mounted alone for a vehicle suspension unit, that is it would require a spring, for example, to maintain the proper height of the part requiring support. Otherwise, unrestricted flow of the hydraulic fluid through the first piston bore would lead to the piston dropping down and coming to rest on the fixed wall.
U.S. Pat. No. 3,817,566 to Keijzer et al (issued June, 1974) discloses an energy absorber wherein a three-position valve assembly is deformed by fluid pressure developed when an external force acts on a piston. A resulting substantial valve deformation enables gas to pass therethrough thereby pressurizing a housing. Once impact is over, the gas bleeds back through the now less-deformed valve to restore the bumper to its original position. The Keijzer device is suited for vehicle bumpers and the like for absorbing energy generated by collisions under about 16 km/h (10 mph).
U.S. Pat. No. 4,506,869 to Masclet et al (issued March, 1985) discloses an aircraft hydro-pneumatic shock absorber having a hydraulic chamber containing a piston and a fixed partition wall. The piston is provided with a fairly large orifice freely communicating oil from said chamber to a second chamber integral with the piston and containing partly oil and partly air, the latter providing a known predetermined spring function. The wall has a constriction valve and across the wall from the piston there is provided a floating piston separating the oil from a pneumatic chamber, thereby providing a second spring function. The latter function may be varied by means of a computer-controlled movable head in the pneumatic chamber, which has the effect of lengthening and shortening the shock absorber, thereby providing an electronically variable force-plunging function for specifically adapting to different aircraft load conditions and take-off and landing absorber roles.
The Masclet unit would require constantly monitoring for hydraulic fluid leakages, since an eventual loss of oil would also render the pneumatic part unoperative
Other known references in the field comprise the following U.S. Patents:
U.S. Pat. No. 2,410,176 (Magnum) PA0 U.S. Pat. No. 3,801,087 (Akaike et al) PA0 U.S. Pat. No. 3,865,356 (Wussner) PA0 U.S. Pat. No. 4,423,801 (Miura) PA0 U.S. Pat. No. 4,687,187 (Bellapadrona) PA0 U.S. Pat. No. 4,795,009 (Tanahashi et al) PA0 U.S. Pat. No. 4,821,983 (Aubry et al) PA0 U.S. Pat. No. 5,330,132 (Lucienne et al)
The disclosures of the above-mentioned U.S. patents are hereby incorporated herein by reference.